In the manufacture of microelectronic devices, photolithographic techniques are commonly utilized. To obtain greater resolution in the formation of microstructures than can be obtained with visible light wavelengths, efforts have been made to use shorter wavelength radiation, particularly X-rays. To achieve adequate resolution, for example, 0.25 micron lithography, the beam of X-rays must display high spectral and spatial uniformity at the plane of the wafer being exposed. Synchrotrons are particularly promising X-ray sources for lithography because they provide a very stable and well defined source of X-rays. The electrons orbiting inside the vacuum enclosure of the synchrotron emit electromagnetic radiation as they are bent by the magnetic fields used to define the path of travel. This electromagnetic radiation is an unavoidable consequence of changing the direction of travel of the electrons and is typically referred to as synchrotron radiation. The energy that the electrons lose in the form of synchrotron radiation must be regained at some point in their orbit around the ring, or they will spiral in from the desired path and be lost. Orbiting electrons can also be lost through collisions with residual gas atoms and ions within the vacuum chamber. Thus, ultra-high quality vacuums are necessary to obtain satisfactory lifetimes of the stored beam.
The synchrotron radiation is emitted in a continuous spectrum of "light", ranging from radio and infrared wavelengths upwards through the spectrum, without the intense, narrow peaks associated with other sources. The shape of a spectral curve of a representative synchrotron storage ring, the Aladdin ring, is shown in FIG. 1. All synchrotrons have similar curves that define their spectra, which vary from one another in intensity and the critical photon energy. The critical photon energy E.sub.c is determined by the radius of curvature of the path of the electrons and their kinetic energy and is given by the relationship: ##EQU1## where R.sub.m is the bending radius, m.sub.e is the electron's rest mass, h is Plank's constant, E.sub.e is the energy of the electron beam and c is the speed of light. Half of the total power is radiated above the critical energy and half below. The higher the kinetic energy of the electrons, or the steeper the bend of the orbit, the higher the critical photon energy. By knowing this information, the synchrotron can be designed to match the spectral requirements of the user.
Parameters describing the size of the source of synchrotron radiation and the rate at which it is diverging from the source are also of importance. Since the electrons are the source of synchrotron radiation, the cross section of the electron beam defines the cross section of the source. Within the plane of the orbit the light is emitted in a broad, continuous fan, which is tangent to the path of the electrons, as illustrated in FIG. 2--which shows a section of a synchrotron 20 having an orbiting electron beam 21 and a fan of synchrotron radiation indicated by the arrows 22. FIG. 3 shows the distribution of the flux of the synchrotron radiation at a plane perpendicular to the plane of the ring, with the distribution of flux indicated by the density of the dots shown within the box 25 in FIG. 3. The flux is substantially uniform horizontally, as shown in the graph at 26, and exhibits a Gaussian distribution profile vertically as shown by the graph 27 in FIG. 3.
Because of the relatively small height and width of the electron beam, it acts as a point source of radiation, providing crisp images at an exposure plane which is typically 8 meters or more away from the ring. However, at a distance of 8 meters a one inch wide exposure field typically collects only 3.2 milli-radians of the available radiation. There are two ways to improve the power incident at the photo-resist: either shorten the beamline or install focusing elements. The use of focusing elements has the potential advantage of collecting X-rays from a very wide aperture and providing a wide image with a very small vertical heigth. However, the use of focusing elements results in a loss of power at each element because of low reflectivity of the X-rays and introduces aberrations. To operate within acceptable values of reflectivity and maximize the delived power, it is necessary to work at grazing angles (i.e., at angles of incidence .theta. from a normal to the surface such that 86.degree..ltoreq..theta..ltoreq.89.5.degree.). Furthermore, because synchrotron radiation is emitted in a horizontal fan, the use of grazing incidence optics is particularly suitable. The small vertical divergence of the synchrotron radiation implies that a wide horizontal mirror can accept a large fan of light at a small grazing angle without being unacceptably long.
The optical system (beamline) must deliver uniform power over the exposure area, typically 2 inches horizontally by 1 inch vertically. This can be achieved by (a) expanding the X-ray beam or (b) scanning the X-ray beam across the image. The first approach is not compatible with vacuum isolation. The present invention is well suited to the second approach, both in the form of mask-wafer scanning and beam rastering.
An X-ray lithography beamline suitable for production purposes should deliver a stable and well characterized flux of X-rays to the exposure field. Desirable characteristics for an X-ray lithography beamline for production purposes include uniform power density over the entire scan region, large collection angle near the source, minimal losses of useful X-rays, a modular optical package with stable, inexpensive recoatable optical elements, and an exposure field measuring at least 1 inch by 1 inch and preferably 2 inches by 2 inches.
Various beamline designs have been proposed for use in X-ray lithography. These include straight-through transmission systems, for example as in B. Lai, et al., "University of Wisconsin X-Ray Lithography Beamline: First Results," Nucl. Instrum. Methods A 246, pp. 681 et seq., (1986); H. Oertel, et al., "Exposure Instrumentation For the Application of X-Ray Lithography Using Synchrotron Radiation," Rev. Sci. Instrum. 60(7), pp. 2140 et seq., 1989. Other systems have utilized planar optics to provide scanning and filtering capabilities. See, H. Betz, "High Resolution Lithography Using Synchrotron Radiation," Nucl. Instrum. Methods A 246, pp. 659 et seq., 1986; P. Pianetta, et al., "X-Ray Lithography and the Stanford Synchrotron Radiation Laboratory," Nucl. Instrum. Methods A 246, pp. 641 et seq., 1986; S. Qian, et al., "Lithography Beamline Design and Exposure Uniformity Controlling and Measuring," Rev. Sci. Instrum. 60(7), pp. 2148 et seq., 1989; E. Bernieri, et al., "Optimization of a Synchrotron Based X-Ray Lithographic System," Rev. Sci. Instrum. 60(7), pp. 2137 et seq., 1989; U.S. Pat. No. 4,803,713 to K. Fuiii entitled "X-Ray Lithography System Using Synchrotron Radiation"; E. Burattini, et al., "The Adone Wiggler X-Ray Lithography Beamline," Rev. Sci. Instrum. 60(7), pp. 2133 et seq., 1989. The use of single figured mirrors is proposed in the article by J. Warlaumont, "X-Ray Lithography in Storage Rings," Nucl. Instrum. Methods A 246, pp. 687 et seq,, 1986. Other proposed systems include the use of Bragg reflections from cystalline surfaces as described in U.S. Pat. No. 4,028,547 entitled "X-Ray Photolithography" and microfabricated structures as described by R. J. Rosser, "Saddle Toroid Arrays: Novel Grazing Incidences Optics for Synchrotron X-Ray Lithography," Blackett Laboratory, Imperial College, London, England.
In an X-ray lithography system, the X-rays are directed through an X-ray mask and onto the photo-resist in those areas which are not shadowed by the non-transmissive pattern formed on the X-ray mask. Generally, the mask will consist of a thin substrate layer which is overlaid by an X-ray absorbing material in the desired pattern. The transmission of the X-ray mask substrate and the absorption of the photo-resist can be used to define the efficiency of the mask/resist system. Low energy X-rays striking the mask substrate are readily absorbed by the substrate material and never make it to the photo-resist. The energy of these absorbed photons goes into heating the mask, which can lead to undesirable side effects as expansion and distortion of the mask. Very high energy X-rays pass through the mask substrate, the absorber, and the photo-resist with few of the interactions that lead to image formation, reducing the usefulness of these photons. On the other hand, those high energy photons that do interact with the photo-resist may have passed through the absorber or "dark" areas of the pattern on the mask, thus reducing the contrast of the image produced in the resist. The product of the mask transmission and the photoresist absorption defines the system response. Thus, it is preferable that the X-ray flux which reaches the X-ray mask be mainly composed of photons which have an energy which lies in an optimal energy region referred to as the "Process Window." The Process Window will vary depending on the mask substrate and the photo-resist choosen, but in general the Process Window will be in the range of 600 eV to 2000 eV, as illustrated in FIG. 4 for the case of the 2 micron thick polycrystalline silicon mask substrate and a 1 micron thick Novolac photo-resist.